Answers

Why not sow in boxes and transplant the little plants? If you remove all the grass it would look OK this year but most of the seed you buy as wildflowers only performs for one year, and then you are left with bare soil.

Oh dear, not really what I was hoping for as I wanted a wildflower mini meadow this summer. last year I cleared an area by hand but want to extend it now and of course the grass has grown over the whole area. I don't know any other way of removing all the grass - the weeds I can dig out but how do you un grass an area ??

Glyphosate will kill grass and everything else it touches the leaves of. Roundup is just a tradename for Glyphosate with something added and is more expensive. Its been found that both will leave residues in the soil.
But as an alternative you could take the turfs off and either stack them face down to rot or bury them in situ a spit deep face down.

Yea, de turf the area - you can hire a machine that will do it for you, then you'd need to turn the soil over underneath before seeding. Otherwise, use a turfing iron, or a sharp spade, or a half moon edger - if using the spade or edger, cut the turf into manageable small sections, penetrating the soil about an inch to inches deep, then lever out the cut sections. Then turn the soil over...

Another 'chemical free' method is called 'lasagna' gardening. Take big flats of cardboard, or several sheets of newspaper and completely cover the area you want to de-turf. Wet it down real good, making sure the layers are thick enough to block all light. The grass will be gone after a couple weeks with no chemicals present.

Well, maybe, the advice is good, but I have to say that when we began here I used Glyphosate to kill off the weeds and planted as soon as the top growth was good and dead. Not on Veg patch I hasten to add.
And yes you would still need to remove the top layer. Most wild flower mixes do far better in poorer soil and if your lawn has been properly maintained, the top 2 or 3 inches are going to be too fertile for the wild flowers.

I think the soil is quite fertile [how would I know ?] it's full of bugs and worms etc. and gets a lot of umm, manure ? [cats and foxes]. The wild flower seed all did very well last year - except, strangely the yellow rattle which I was hoping would inhibit the grass a bit.

Glyphosate does not kill micro organisms in soil - where it does kill both micro organisms and fungii is in soil-less mediums (so things like multi purpose potting compost, but not John Innes). If anyone wants to read the science, its in the link below

I may have missed the point but if you are hoping for a wildflower meadow you probably don't need to kill the grass. Without grass it will be a flower bed rather than a meadow! Or are you planting with a specific meadow grass as well?

No just a mix of varied flower seeds, but the tough grass seems to swamp them and not give the seeds a chance to sprout up. I put lots of yellow rattle seeds out among the grass but none have sprouted.

Wondering how you decided to deal with the grasses? I have been doing several "reclamation" areas -- in late spring or summer, water area, lay cardboard on top of whatever is growing (including Himalayan blackberries & bindweed) put a foot or more of wood chips. Wait until fall or next spring to plant (mix a small bit of soil, and broadcast over wood chips).